So you submit your GPA that’s the average of freshman, sophomore, junior, and senior (however much you’ve completed) grades?
Yes - if you are applying in the fall of your senior year, you will have 3 years of grades (freshman, sophomore, junior). If using the common application, there is a form for your school to send mid-year grades. Also, on the common app, you will list what classes you are taking senior year.
You need to ask your HS counselor how/when your school calculates GPA. My HS did not recalculate GPA in the middle of senior year (even if a class was completed) but other schools may.